Background: It is known that children are more physically active outdoors than indoors. However, few previous studies have observed the time course for physical activity as young children transition from indoor to outdoor activities. Methods: Participants were 3-to 5-year-old children enrolled in the Children's Activity and Movement in Preschool Study (CHAMPS). Trained observers used the Observational System for Recording Physical Activity in Children-Preschool Version (OSRAC-P) to record children's physical activity levels over 20 minutes in outdoor settings. The 20-minute outdoor observational period began immediately following the transition from indoors to outdoors. Results: Children's activity levels were moderately high at the time of transition and declined over the 20-minute observation period. Different patterns, however, were observed for boys and girls. Overall, boys were more active than girls. Boys' activity levels declined in a linear fashion over the 20-minute period, while girls' activity levels increased slightly, decreased, and then increased slightly again. Conclusions: These data indicate that physical activity levels decline with increased duration of outdoor play. The frequency and duration of outdoor play should be investigated for the purpose of optimizing physical activity levels. (author abstract)